OPA354-Q1_15 Datasheet, PDF (21/36 Pages) Texas Instruments – OPAx354-Q1 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers

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OPA354-Q1, OPA2354-Q1, OPA4354-Q1
SBOS492B – JUNE 2009 – REVISED DECEMBER 2014
8.2.3 Driving ADCs
The OPAx354-Q1 op amps are well-suited for driving sampling analog-to-digital converters (ADCs) with sampling
speeds up to 1 MSPS. The zero-crossover distortion input stage topology allows the OPAx354-Q1 family of
devices to drive ADCs without degradation of differential linearity and THD.
The OPAx354-Q1 family of devices can be used to buffer the ADC switched input capacitance and resulting
charge injection while providing signal gain. Figure 41 shows the OPAx354-Q1 family of devices configured to
drive the ADS8326.
5V
V_I
0 to 4.096 V
C1
100 nF
V_(V+)
OPAx354-Q1
V_(V–)
R1⁽¹⁾
100 Ω
C3⁽¹⁾
1 nF
–5 V
R2
50 kΩ
Optional⁽²⁾
SD1
BAS40
C2
100 nF
5V
REF3240
4.096 V
5V
+INx
–INx
ADS8326
16-Bit
250kSPS
REF IN
C4
100 nF
(1) Suggested value; may require adjustment based on specific application.
(2) Single-supply applications lose a small number of ADC codes near ground as a result of op amp output swing limitation. If a negative
power supply is available, this simple circuit creates a –0.3-V supply to allow output swing to true ground potential.
Figure 41. Driving the ADS8326
8.2.4 Active Filter
The OPAx354-Q1 family of devices is well-suited for active filter applications that require a wide bandwidth, fast
slew rate, low-noise, single-supply operational amplifier. Figure 42 shows a 500 kHz, second-order, low-pass
filter using the multiple-feedback (MFB) topology. The components have been selected to provide a maximally-
flat Butterworth response. Beyond the cutoff frequency, roll-off is –40 dB/dec. The Butterworth response is ideal
for applications requiring predictable gain characteristics, such as the anti-aliasing filter used in front of an ADC.
One point to observe when considering the MFB filter is that the output is inverted, relative to the input. If this
inversion is not required, or not desired, a noninverting output can be achieved through one of the following
options:
1. Adding an inverting amplifier
2. Adding an additional second-order MFB stage
3. Using a noninverting filter topology, such as the Sallen-Key (see Figure 43).
MFB and Sallen-Key, low-pass and high-pass filter synthesis is quickly accomplished using TI’s FilterPro™
program. This software is available as a free download at www.ti.com.
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